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DESIGN AND ANALYSIS OF COLLUSION-RESISTANT
FINGERPRINTING SYSTEMS
by
Byung-Ho Cha
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(ELECTRICAL ENGINEERING)
August 2009
Copyright 2009 Byung-Ho Cha

The design and analysis of a collusion-resistant fingerprinting system for traitor tracing applications in a large-scale distribution scenario is proposed in this research. One key issue in the fingerprinting system design is its robustness against the collusion attacks. Our main research stream exactly follows that the task of identifying colluders and their weights can be formulated as a user detection and channel estimation problem in a multiuser wireless communication. By exploiting this analogy, we extend our research to various directions, and those results are organized through six chapters.; In Chapter 3, we design good fingerprint codes that are effective in resisting collusion attacks in the fingerprinting system. Specifically, we consider a hiding code of MN bits, where M bits are used as the user ID (or message) and N bits are the length of spreading codes. Since each user is assigned one out of 2^(M-1) ID numbers and one out of N spreading codewords, the total number of users is equal to 2^(M-1)N. To accommodate even more users, we propose to reuse the same codeword via circular shift. By allowing P circular shifts, the total number of users increases from 2^(M-1)N to 2^(M-1)NP.; In Chapter 4, collusion-resistance can be achieved by choosing specific spreading codes followed by the multi-carrier modulation. Examples of spreading codes include Hadamard-Walsh codes and carrier interferometry codes. For this reason, our fingerprint codes are called multi-carrier collusion-resistant codes. It is proved that the multiple access interference of users with different spreading codewords can be significantly reduced. Second, we propose the use of channel estimation/equalization and advanced detection schemes to deal with a large number of colluders with weighted collusion attacks in the fingerprinting system. They include the frequency-domain equalization, the maximal ratio combining schemes for colluder detection in the same codeword group, and the multiuser detection techniques for colluder detection in different codeword groups. It is demonstrated by computer simulation that, as compared with previous work in this field, colluders and their attack weights can be found by the proposed fingerprinting system more accurately.; In Chapter 5, the design of robust fingerprinting systems for traitor tracing against time-varying collusion attacks in protecting continuous media, such as audio and video, is investigated. We first show that it can be formulated as a multi-user detection problem in a wireless communication system with a time-varying channel response. Being inspired by the multi-carrier code-division-multi-access (MC-CDMA) technique, we propose a fingerprinting system that consists of three modules: 1) codeword generation with a multi-carrier approach, 2) colluder weight estimation, 3) advanced message symbol detection. We construct embedding codes with code spreading followed by multi-carrier modulation. For colluder weight estimation, we show that the weight estimation is analogous to channel response estimation, which can be solved by inserting pilot signals in the embedded fingerprint. As to advanced message symbol detection, we replace the traditional correlation-based detector with the maximal ratio combining detector and the parallel interference cancellation multiuser detector. The superior performance of the proposed fingerprinting system in terms of number of users/identified colluders and the bit error probability of symbol detection is demonstrated by representative audio and video examples.; In Chapter 6, we analyze time-varying collusion attacks over continuous multimedia, e.g., music and movie contents. We introduce the concept of fingerprint-to-interference-plus-noise ratio. We also characterize the capacity of a host media with the just-noticeable-difference based on the human perceptual system model. Based on those formulations, we investigate the relationship between time-varying collusion attacks and multimedia distortions with the subjective quality measure. We analyze time-varying collusion attacks for a fingerprinting system using concepts of capacity and throughput. Continuous media provide a limited resource for data embedding, which is analogous to the capacity of a wireless communication channel. Furthermore, the collusion attack can be interpreted as a channel shared by multiple users. Based on this analogy, the colluder detection performance can be understood from the viewpoint of throughput. Specifically, we show how to determine instantaneous throughput using the fingerprint-to-interference-plus-noise ratio, and extend it to the total throughput and the averaged throughput over a time interval. Our analysis provides a good framework to the understanding of collusion attacks and ways to enhance the traitor tracing performance of a fingerprinting system.; In Chapter 7, an anti-collusion fingerprinting system is developed to protect media files against time-varying collusion attacks based on the scalar Costa scheme and colluder weight recovery. We treat the host signal as a parallel Gaussian channel and fingerprints as transmitted user signals. We decompose the Gaussian channel into multiple independent sub-channels, and assign different user messages to different sub-channels. Then, colluder weights in collusion attacks can be estimated using pilot symbols at the decoder, and all weights can be estimated and compensated. As a result, the decoding region on the parametric space can be recovered as an original format. It is shown by experimental results that the proposed fingerprinting system has excellent performance in colluder detection.; To preserve the tracing capability of media fingerprinting against various types of collusion attacks, the study of advanced detection and suitable media combining techniques is demanded. Our research can be shed lights on the practical and feasible solutions in the design of future fingerprinting systems.

DESIGN AND ANALYSIS OF COLLUSION-RESISTANT
FINGERPRINTING SYSTEMS
by
Byung-Ho Cha
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(ELECTRICAL ENGINEERING)
August 2009
Copyright 2009 Byung-Ho Cha